1. Field of the Invention
This invention relates in general to apparatus for positioning a magnetic transducer with respect to a magnetic track; more particularly, the invention is concerned with a magnetic head positioning and playback apparatus which uses the playback signal to maintain track position.
2. Description Relative to the Prior Art
While not so restricted, the invention acquires a special significance when it is used to accurately position a rotating playback head with respect to a magnetic tape track which was recorded with a transverse or helical scan magnetic tape recorder. In order to realize a high track density, present transverse and helical recording techniques provide a pattern of magnetic tracks with narrow guard bands separating the tracks. The advantages of such a high track density depend, of course, on the ability of the playback head to accurately correlate with each recorded track during reproduction. To maintain the highest correlation, the center of the useful gap of the playback head must invariably follow the center of each recorded track. If the playback head fails to center on each recorded track, interference voltages are established when the playback head slides into the adjacent guard band. In addition, the information output of the head will suffer as it no longer tracks the full recorded track width. Indeed, the information content of the reproduced signal may be lost if the playback head crosses over between recorded tracks.
Endeavoring to meet these difficulties, and in particular for use with helical or transverse recording with rotating heads, one commonly used tracking system disposes a control track adjacent a longitudinal edge of the tape. During playback a control track head, coupled to control circuits in the reproduction electronics, reproduces a timing signal from the control track. This timing signal was earlier recorded simultaneously with the information signal and, as such, reflects the electrical and mechanical transients present during recording. The control circuits compare the timing signal to a feedback pulse from the head drum and generate a tracking signal which so variably drives the rotating heads that they accurately follow the recorded tracks. Another tracking system provides reference tracks on either side of each main track. These reference tracks are scanned by checking heads positioned on either side of a main head; a comparison between the voltages induced in each of the checking heads provides an indication of head to track orientation. Unfortunately, but necessarily, both of the tracking methods thus far described must sacrifice usable portions of the recording surface for placement of reference tracks.
Therefore it is clearly more advantageous, spacewise, to correlate the position of the reporduce head to the magnetic track by using the information signal written in the track itself rather than by using additional reference signal tracks. To this end, magnetic heads have been provided with two auxiliary checking heads having useful checking gaps located on either side in front of and behind the main gap. These gaps are positioned symmetrically with regard to the center of the main gap and each cover an edge of the track. If the main gap is correctly positioned with respect to the track and the magnetization in the track is constant, the voltages induced in each of the side checking heads will be equal to one another, thereby indicating proper orientation. However, this adjusting mechanism requires two auxiliary heads in addition to the main head. For use with a magnetic disc file, Stanley (U.S. Pat. No. 3,246,307) reduces the foregoing apparatus to two separate magnetic elements which are spaced from each other. These elements are effectively separate heads which each correlate with less than half of the magnetic track for producing a servo control signal and, after servo positioning is completed, for producing a playback signal.
In a further development directed toward track control without separate reference tracks, Walther (U.S. Pat. No. 3,541,270) provides a magnetic head having one of its two pole pieces divided in the direction of travel of the magnetic carrier into two magnetically insulated portions. A coil is provided for each of the portions to allow separate voltages representative of head position to be generated. By relative comparison of the two voltages induced in each of the portions during the adjustment of the head of the track, any positional error may be directly detected. The amplitude of the difference signal thus detected is a measure of positional error while the phase of the difference signal will indicate the direction of deviation of the head from the track. Once correctly adjusted to the track, the polarity of one of the checking circuits may be altered and the head may be used as a conventional playback head across a full track width. While this system may be used for intermittent playback from, for example, a random access magnetic disc assembly, it is unsuited for maintaining continuous track control over, for example, helically-recorded video information being reproduced from a magnetic tape.